In a medium bowl, add the blackberries, sugar, cornstarch, lemon juice and zest, and salt. Toss to combine. Transfer to a 9x5-inch loaf pan.
Make the topping by adding the flour, coconut, sugar, hazelnuts, baking powder, and salt in a large bowl. Cut in the butter using a pastry blender until pea-sized crumbs form. Add the egg and and stir until a dough forms.
Flatten about 3 tablespoons of dough and place on top of the berry filling. Continue with the remaining dough until the top is roughly covered.
Bake for 35 to 40 minutes until the topping is golden brown and the filling is bubbly. Allow to cool on a cooling rack for 20 minutes before serving.